Double clutch transmission for a hybrid electric vehicle and method for operating the same

ABSTRACT

A double clutch transmission is described that ideally changes torque received from an engine and a motor by providing an optimal layout in power transmission scheme and motor location. Such a double clutch transmission is optimally adapted to a hybrid electric vehicle, and an optimal operation method for such a double clutch transmission is also provided, overcoming inefficiency in application of a conventional CVT to an HEV.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.11/023,700, filed Dec. 27, 2004, now U.S. Pat. No. 7,249,537 whichclaims priority to Korean Application No. 10-2003-0096568, filed on Dec.24, 2003, the contents of both of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to a double clutch transmission for ahybrid electric vehicle, and a method for operating the same.

BACKGROUND OF THE INVENTION

Generally, “hybrid vehicle” is a vehicle utilizing a plurality of powersources, and usually refers to a hybrid electric vehicle (HEV) that isdriven by an engine and a motor. HEVs may be realized in various schemesadopting an engine and a motor, and a majority of schemes are based on aparallel construction or a series construction.

A series scheme is simpler in structure than a parallel scheme, so it iseasier to control. However, series HEVs are less energy efficient thanparallel HEVs because energy in series HEVs is first converted frommechanical energy of an engine into electrical energy in a battery, andthen used for running a motor. Parallel HEVs, though more complicated instructure and control, are more energy efficient because mechanicalenergy of an engine and electrical energy of a battery may besimultaneously used. For this reason, a parallel scheme is usuallyadopted for a passenger car.

A series HEV, though less energy efficient than a parallel HEV, canalways operate the engine at an optimal operating point. However, aparallel HEV cannot always operate the engine at an optimal operatingpoint since the engine and the motor are mechanically coupled togetherthrough a transmission and the engine speed is correlated with thevehicle speed. Consequently, operating efficiency of an engine variesaccording to vehicle speed. In order to solve this problem, acontinuously variable transmission (CVT) utilizing a metal belt isusually considered a favorable transmission because it enables theengine speed to be controlled independently from the vehicle speed.However, such a CVT requires very high hydraulic pressure for operationin comparison with other transmissions such as an automatictransmission. Therefore, in spite of various functionally favorablefeatures, a CVT does not manifest particularly substantial energyefficiency.

On the other hand, in a double clutch transmission (DCT), torque from anengine is transmitted to two input shafts through two clutches, and isthen changed and output using gears associated with the two inputshafts. By adapting two clutches and an automatically shifting device toa scheme similar to a conventional manual transmission, the convenienceof an automatic transmission may thusly be obtained while maintainingthe efficiency level of a manual transmission.

Therefore, if such a DCT may be adapted to an HEV as its transmissionsystem, the HEV's efficiency may be further enhanced from an HEV thatuses a CVT.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the background of theinvention, and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art that is alreadyknown in this country to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

The present invention provides a double clutch transmission for a hybridelectric vehicle, and an operation method structured to provideadvantages of ideally changing and outputting torque of an engine and amotor.

An exemplary double clutch transmission for a hybrid electric vehiclehaving an engine and a motor according to an embodiment of the presentinvention includes a main input shaft, first and second input shafts,first and second clutches, a drive gear unit, first and second outputdevices, a differential gear, and a motor input/output unit.

The main input shaft receives torque from the engine. The first inputshaft rotates coaxially with the main input shaft. The second inputshaft rotates coaxially with the main input shaft and along an exteriorcircumference of the first input shaft. The first and second clutchesselectively transmit a torque from the main input shaft to the first andsecond input shafts.

The drive gear unit includes a plurality of drive gears disposedrespectively on the first and second input shafts.

The first output device includes a first output shaft disposed parallelto and apart from the first and second input shafts by a predetermineddistance, and also includes a plurality of driven gears and a firstoutput gear thereon, such that torque of drive gears on the first andsecond input shafts are selectively changed and output.

The second output device includes a second output shaft and a reverseidle shaft disposed parallel to and apart from the first and secondinput shafts by predetermined distances, a plurality of driven gears, asecond output gear, a reverse driven gear disposed on the second inputshaft, and a plurality of reverse mediating gears disposed on thereverse idle shaft, such that torque of drive gears on the first andsecond input shafts are selectively changed and output.

The differential gear is commonly connected to the first output gear andthe second output gear.

The motor input/output unit selectively transmits a torque of the motorto the second input shaft through a plurality of gears and a drive gearon the second input shaft.

In another embodiment, the drive gear unit includes first, third, andfifth drive gears formed on one input shaft among the first and secondinput shafts, and second, fourth, and sixth drive gears formed onanother input shaft among the first and second input shafts.

In a further embodiment, the first, third, and fifth drive gears areformed on the first input shaft, and the second, fourth, and sixth drivegears are formed on the second input shaft.

In yet another embodiment, the first, second, third, fourth, fifth, andsixth drive gears are disposed in a sequence of the second drive gear,the fourth drive gear, the sixth drive gear, the third drive gear, thefirst drive gear, and the fifth drive gear from the engine.

In yet a further embodiment, the first output device includes the firstoutput shaft; first, second, third, and fourth driven gears; first andsecond synchronizing devices; and the first output gear. The first,second, third, and fourth driven gears are disposed on the first outputshaft and are respectively engaged with the first, second, third, andfourth drive gears. The first synchronizing device selectively transmitstorque of the first and third driven gears to the first output shaft.The second synchronizing device selectively transmits a torque of thesecond and fourth driven gears to the first output shaft. The firstoutput gear is disposed on the first output shaft and engaged with thedifferential gear. Accordingly, torque of the first, second, third, andfourth drive gears on the first and second input shafts are selectivelychanged and output.

In another further embodiment, the second output device includes thefirst output shaft, the reverse idle shaft, fifth and sixth drivengears, first and second mediating gears, a reverse driven gear, thirdand fourth synchronizing devices, and the second output gear. The fifthand sixth driven gears are disposed on the second output shaft andrespectively engaged with the fifth and sixth drive gears. The firstmediating gear is disposed on the reverse idle shaft and is engaged withthe first drive gear. The second mediating gear is disposed on thereverse idle shaft. The reverse driven gear is disposed on the secondoutput shaft and is engaged with the second mediating gear. The thirdsynchronizing device selectively transmits a torque of the fifth drivengear to the second output shaft. The fourth synchronizing deviceselectively transmits torque of the sixth and reverse driven gears tothe second output shaft. The second output gear is disposed on thesecond output shaft and is engaged with the differential gear.Accordingly, torque of the first, fifth, and sixth drive gears on thefirst and second input shafts are selectively changed and output.

In yet a further embodiment, the motor input/output unit includes amotor gear, a motor idle shaft, and a motor idle gear. The motor gear isdisposed on a rotation shaft of the motor. The motor idle shaft isdisposed parallel to and apart from the second input shaft by apredetermined distance. The motor idle gear is formed on the motor idleshaft and is commonly engaged with the motor gear and one drive gear onthe second input shaft. In this embodiment, the one drive gear on thesecond input shaft engaged with the motor idle gear may be a drive gearfor a second speed.

An exemplary method for controlling a double clutch transmission is amethod for controlling a double clutch transmission for a hybridelectric vehicle. The method includes determining an operation mode froman electric vehicle (EV) mode, a hybrid electric vehicle (HEV) mode, anda regenerative braking (RB) mode. The double clutch transmission isoperated in accordance with the determined operation mode. In the EVmode, only the torque of the motor is changed and output. In the HEVmode, torque of both the motor and the engine are changed and output. Inthe RB mode, braking and inertial energy of the hybrid electric vehicleis reclaimed by electrical generation of the motor.

In a further embodiment, the EV mode is determined when a current stateof charge (SOC) is above a predetermined SOC. In the EV mode, the engineis stopped and the first and second clutches are released while a torqueof the motor is transmitted to a driven gear for a predeterminedshift-speed, such that only the torque of the motor from battery poweris changed and output. In a further embodiment, the predeterminedshift-speed is a forward second speed.

In another further embodiment, in the EV mode, power of the motor isoutput to the differential gear sequentially through a motor gear, amotor idle gear, a second speed drive gear on the second input shaft, asecond speed driven gear on the first output shaft, and the first outputshaft.

In yet another embodiment, in the HEV mode, the engine is started by themotor, either the first or second clutches is engaged such that theengine's torque is transmitted to a driven gear for a targetshift-speed, the motor's torque is transmitted to a second speed drivengear, and motor's torque is controlled based on a current vehicle speedand a depression amount of an accelerator pedal.

In another further embodiment, the RB mode is when the hybrid electricvehicle is decelerating by braking or is inertially running. In the RBmode, the first and second clutches are released while torque istransmitted from a driven gear for a predetermined shift-speed to themotor, such that the motor is driven as an electric generator by thebraking and inertial energy of the hybrid electric vehicle.

In yet another embodiment, in the RB mode, the braking and inertialenergy are input to the motor sequentially through the differentialgear, the first output gear, the first output shaft, a second speeddriven gear on the first output shaft, a second speed drive gear on thesecond input shaft, the motor idle gear, and the motor gear.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate an embodiment of the invention,and, together with the description, serve to explain the principles ofthe invention, wherein:

FIG. 1 is a schematic diagram of a DCT for an HEV according to anembodiment of the present invention;

FIG. 2 is a drawing illustrating a disposition relationship among firstand second input shafts, first and second output shafts, a reverse idleshaft, a differential gear, and a motor gear of a DCT for an HEVaccording to an embodiment of the present invention;

FIG. 3 is a diagram showing an EV mode operation of a DCT for an HEVaccording to an embodiment of the present invention;

FIG. 4 is a diagram showing the operation of a DCT for an HEV accordingto an embodiment of the present invention when an engine is startedwhile the DCT is in EV mode;

FIG. 5 is a diagram showing the operation of a DCT for an HEV accordingto an embodiment of the present invention when the HEV is running at aforward third speed in HEV mode;

FIG. 6 is a diagram showing the RB mode operation of a DCT for an HEVaccording to an embodiment of the present invention; and

FIG. 7 is a flowchart showing a method for controlling a DCT accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will hereinafter be described indetail with reference to the accompanying drawings.

As is well known in the art, a double clutch transmission (DCT) includestwo clutch devices within a transmission scheme that is similar to amanual transmission. In a DCT, torque from an engine is transmitted totwo input shafts through two clutches, and is then changed and outputusing gears disposed on the two input shafts.

An embodiment of the present invention applies such a DCT to a hybridelectric vehicle (HEV) having two power sources of an engine and amotor. FIG. 1 is a schematic diagram of a DCT for an HEV according to anembodiment of the present invention, and FIG. 2 is a drawingillustrating a disposition relationship among first and second inputshafts, first and second output shafts, a reverse idle shaft, adifferential gear, and a motor gear of a DCT for an HEV according to anembodiment of the present invention.

As shown in FIG. 1, a DCT for an HEV according to an embodiment of thepresent invention includes a main input shaft 105; first and secondinput shafts 110 and 120; first and second clutches C1 and C2; first,second, third, fourth, fifth, and sixth drive gears G1, G2, G3, G4, G5,and G6; first and second output devices OUT1 and OUT2; differential gearDIFF; and a motor input/output unit 190.

The main input shaft 105 receives torque from an engine 102. The firstinput shaft 110 is rotatably disposed along a rotation axis of the maininput shaft 105. The second input shaft 120 is disposed on the rotationaxis of the main input shaft 105 rotatably around the first input shaft110. The first and second clutches C1 and C2 selectively transmit thetorque of the main input shaft 105 to the first and second input shafts110 and 120, respectively. Therefore, the torque of the main input shaft105 is transmitted to the first input shaft 110 when the first clutch C1operates, and the torque of the main input shaft 105 is transmitted tothe second input shaft 120 when the second clutch C2 operates. Thefirst, third, and fifth drive gears G1, G3, and G5 are formed on thefirst input shaft 110, and the second, fourth, and sixth drive gears G2,G4, and G6 are formed on the second input shaft 120. Further, the first,third, and fifth drive gears G1, G3, and G5 are formed on the firstinput shaft 110 such that the third drive gear G3 is close to an end ofthe second input shaft 120, the fifth drive gear G5 is distal thereto,and the first drive gear G1 is between the third and fifth drive gearsG3 and G5.

Additionally, the second, fourth, and sixth drive gears G2, G4, and G6are formed on the second input shaft 120 such that the second drive gearG2 is close to the engine 102, the sixth drive gear G6 is distal to theengine 102, and the fourth drive gear G4 is between the second and sixthdrive gears G2 and G6. Therefore, according to an embodiment of thepresent invention as shown in FIG. 1, the first, second, third, fourth,fifth, and sixth drive gears G1, G2, G3, G4, G5, and G6 are arranged inthe following sequential order: the second drive gear G2, the fourthdrive gear G4, the sixth drive gear G6, the third drive gear G3, thefirst drive gear G1, and then the fifth drive gear G5.

In addition, as shown in FIG. 1, the DCT includes a first output deviceOUT1 for selectively changing and outputting torque of the first,second, third, and fourth drive gears G1, G2, G3, and G4; and a secondoutput device OUT2 for selectively changing and outputting torque of thefirst, fifth, and sixth drive gears G1, G5, and G6.

As shown in FIG. 1, the first output device OUT1 includes a first outputshaft 130; first, second, third, and fourth driven gears D1, D2, D3, andD4; first and second synchronizing devices S1 and S2; and a first outputgear 135. The first output shaft 130 is disposed parallel to and apartfrom the main input shaft 105 by a predetermined distance. The first,second, third, and fourth driven gears D1, D2, D3, and D4 are disposedon the first output shaft 130 while being respectively engaged with thefirst, second, third, and fourth drive gears G1, G2, G3, and G4. Thefirst synchronizing device S1 selectively transmits torque from eitherthe first or third driven gears D1 and D3 to the first output shaft 130.The second synchronizing device S2 selectively transmits torque fromeither the second or fourth driven gears D2 and D4 to the first outputshaft 130. In addition, the first output gear 135 is disposed on thefirst output shaft 130 while being engaged with the differential gearDIFF such that torque received from the first, second, third, and fourthdrive gears G1, G2, G3, and G4 is transmitted to the differential gearDIFF.

As shown in FIG. 1, the second output device OUT2 includes a secondoutput shaft 140, a reverse idle shaft 150, fifth and sixth driven gearsD5 and D6, first and second mediating gears M1 and M2, a reverse drivengear R, third and fourth synchronizing devices S3 and S4, and a secondoutput gear 145. The second output shaft 140 and the reverse idle shaft150 are disposed parallel to and apart from the main input shaft 105 bypredetermined distances. The fifth and sixth driven gears D5 and D6 aredisposed on the second output shaft 140 while being respectively engagedwith the fifth and sixth drive gears G5 and G6. The first mediating gearM1 is disposed on the reverse idle shaft 150 while being engaged withthe first drive gear G1. The reverse driven gear R is disposed on thesecond output shaft 140 while being engaged with the second mediatinggear M2. The third synchronizing device S3 selectively transmits thetorque of the fifth driven gear D5 to the second output shaft 140. Thefourth synchronizing device S4 selectively transmits torque from eitherthe reverse driven gear R or the sixth driven gear D6 to the secondoutput shaft 140. In addition, the second output gear 145 is disposed onthe second output shaft 140 while being engaged with the differentialgear DIFF, such that a torque received from the first, fifth, and sixthdrive gears G1, G5, and G6 is transmitted to the differential gear DIFF.

Details of the first, second, third, and fourth synchronizing devicesS1, S2, S3, and S4 may be obviously understood by a person of ordinaryskill in the art, referring to synchronizing devices of a conventionalmanual transmission operated by a shift fork. For example, the first,second, third, and fourth synchronizing devices S1, S2, S3, and S4 maybe respectively operated by additional actuators controlled by acontroller (not shown), in left and right directions in FIG. 1. Theactuator may be driven by an electric motor or hydraulically driven by asolenoid valve controlling hydraulic pressure from an oil pump. Thesedetails are obvious to a person of ordinary skill in the art, andaccordingly are not described in further detail herein.

The motor input/output unit 190 transmits the torque of a motor 103(i.e., a motor-generator that can be operated for driving and generatingfunctions), disposed to a transmission case (not shown), selectively tothe second input shaft 120 through a motor gear MG, a motor idle gearIG, and the second drive gear G2 on the second input shaft 120. Themotor gear MG is disposed on the motor 103 on its rotation shaft 170.The motor idle shaft 160 is disposed parallel to and apart from thesecond input shaft 120 by a predetermined distance. On the motor idleshaft 160, a motor idle gear IG is disposed while also being engagedwith the motor gear MG and the second drive gear G2 on the second inputshaft 120.

In FIG. 1, the engagement of the first mediating gear M1 and the firstdrive gear G1 and the engagement of the second output shaft 140 and thedifferential gear DIFF are shown by dotted lines. This is because thefirst and second input shafts 110 and 120, the first and second outputshafts 130 and 140, the reverse idle shaft 150, and the differentialgear DIFF are planarly drawn for illustrational convenience, althoughthey are spatially disposed. Such a spatial disposition of the first andsecond input shafts 110 and 120, the first and second output shafts 130and 140, the reverse idle shaft 150, and the differential gear DIFFappears in FIG. 2.

While FIG. 1 is a schematic view of the DCT from above, FIG. 2 is aschematic view of the DCT from the left. Further, some gears shown inFIG. 5 are intentionally not shown in FIG. 2 for better understanding ofthe spatial relationship among rotation axes of rotating elements.

As shown in FIG. 2, the first and second output shaft 130 and 140 aredisposed apart from the second input shaft 120. The reverse idle shaft150, first input shaft 110, and second input shaft 140 are positionedsuch that their centers form three points of a triangle. The firstmediating gear M1 on the idle shaft 150 is engaged with the first drivegear G1 of the first input shaft 110, and the second mediating gear M2on the idle shaft 150 is engaged with the reverse driven gear R of thesecond output shaft 140. The differential gear DIFF and the first andsecond output shafts 130 and 140 are positioned such that their centersform three points of a triangle. Further, the differential gear DIFF isengaged with the first and second output gears 135 and 145 of the firstand second output shafts 130 and 140. finally, the motor idle gear IG onthe motor idle shaft 160 is engaged with the second drive gear G2 of thesecond input shaft 120 and the motor gear MG of the motor rotation shaft170.

With such a DCT for an HEV, disposition of six drive gears on inputshafts may enable a total of seven speeds (i.e., six forward speeds andone reverse speed). The shifting operations of such a DCT according to afirst embodiment of the present invention will now be described indetail.

As can be seen in FIG. 1, to shift into the first speed, the firstdriven gear D1 and the first output shaft 130 are synchronously engagedby operating the first synchronizing device S1, and then operating thefirst clutch C1. To shift into the second speed, the second driven gearD2 and the first output shaft 130 are synchronously engaged by operatingthe second synchronizing device S2 while the first clutch C1 operatesand the second clutch C2 is released, and then releasing the firstclutch C1 and operating the second clutch C2. The first synchronizingdevice S1 is moved to a neutral position such that the first driven gearD1 and the first output shaft 130 are disengaged.

Similar to shifting into the first and second speeds, shifting to thethird, fourth, fifth, sixth, and reverse speeds, involves synchronouslyengaging a corresponding driven gear and a corresponding output shaft byoperating a corresponding synchronizing device while the first andsecond clutches C1 and C2 are alternately engaged. In addition, adjacentspeeds require different synchronizing devices to be operated.Therefore, a release of a current speed and a realization of a targetspeed may be independently controlled during shifting from and toadjacent speeds. In addition, during shifting to an adjacent speed,various manipulation techniques that a driver may perform on a manualtransmission (e.g., a half-clutch operation) may be realized bycontrolling engagement timing of an on-coming clutch and release timingof an off-going clutch.

Operation of such a DCT for an HEV is based on modes. Such operationmodes include an electric vehicle (EV) mode wherein only torque of themotor 103 is utilized, a hybrid electric vehicle (HEV) mode whereintorque of the engine 102 is used as a main power source and torque ofthe motor 103 is used as an auxiliary power source, and a regenerativebraking (RB) mode wherein braking and inertial energy of the hybridelectric vehicle is reclaimed by electric generation of the motor 103and is used for recharging a battery (not shown).

In the EV mode, as shown in FIG. 3, the vehicle and the engine 102 arestopped and the first and second clutches C1 and C2 are released. Whenan accelerator pedal is depressed by a driver, the second synchronizingdevice S2 connects the second driven gear D2 and the first output shaft130 when the battery's current state of charge (SOC) is sufficientlyhigh (that is, when a current SOC is above a predetermined SOC). In thiscase, the motor 103 is driven by battery power. Accordingly, startingthe HEV is accomplished by driving the motor 103, and, therefore, theHEV is driven at the second speed by the motor 103. In such an EV mode,power of the motor is output to the differential gear sequentiallythrough a motor gear MG, a motor idle gear IG, a second drive gear G2 onthe second input shaft 120, a second driven gear D2 on the first outputshaft 130, and the first output shaft 130.

When shifting to a specific shift-speed (e.g., to the third speed) isrequired due to an increase of vehicle speed, the engine 102 is started.The vehicle is released from the EV mode and enters the HEV mode. Whenthe engine 102 is required to be started while the DCT is operated inthe EV mode, as shown in FIG. 4, the second clutch C2 is slip-controlledsuch that the engine 102 is started by torque from the motor 103. Thatis, by slip-controlling the second clutch C2, the engine 102 is firstlydriven to an appropriate speed for firing. When the engine is started,the second clutch C2 is then released such that shifting to a targetspeed (e.g., the third speed) may be executed by operating acorresponding synchronizing device.

In the above description, the HEV is described to be started only by thepower of the motor 103 when the current SOC of the battery issufficiently high. When the SOC of the battery is not sufficiently high,the HEV is started and driven by immediately starting the engine 102 andusing the power of the engine 102 as a primary power source. This HEVmode will now be described in detail with reference to FIG. 5, withrespect to an exemplary case that the DCT is driven at the third speedin the HEV mode.

When shifting to the third speed while the HEV is running in EV mode,the engine 102 is started by the torque of the motor 103. The firstsynchronizing device S1 is then coupled with the third driven gear D3and the second synchronizing device S2 remains coupled with the seconddriven gear D2 while the first clutch C1 is engaged and the secondclutch C2 is released. In this case, the HEV is driven primarily by thetorque of the engine 102 and secondarily by the torque of the motor 103.In a further embodiment, the auxiliary power of the motor 103 isdetermined on the basis of an accelerator pedal depression amount of adriver and a current vehicle speed.

Specific values of the auxiliary power of the motor 103 may becalculated and realized by a controller (not shown) according to apredetermined algorithm. They may be arbitrarily set by a person ofordinary skill in the art taking into account the design specificationsof the DCT and/or HEV, and are therefore not described in furtherdetail.

In the case that the hybrid electric vehicle is decelerating by brakingor is inertially running, RB mode is determined, releasing the first andsecond clutches C1 and C2 and coupling the second synchronizing deviceS2 with the second driven gear D2, as shown in FIG. 6. In RB mode,braking and inertial energy is reclaimed and charged into the battery byan electricity generation operation of the motor 103 according to acurrent vehicle speed. In this case, it is preferable that the operationof the motor 103 is controlled within an operation range that is mostefficient for electricity generation. Specifically, such an operationrange and controlling of the motor 103 therewithin with respect to thecurrent vehicle speed may be implemented into and realized by acontroller (not shown). These may be arbitrarily set by a person ofordinary skill in the art taking into account the design specificationof the DCT and/or HEV, and are not therefore described in furtherdetail.

In RB mode, braking and inertial energy is input to the motorsequentially through the differential gear DIFF, the first output gear135, the first output shaft 130, the second driven gear D2 on the firstoutput shaft 130, the second drive gear G2 on the second input shaft120, the motor idle gear IG, and the motor gear MG, which is the reversesequence of power transmission flow in the second speed. Such a methodfor operating a DCT according to an embodiment of the present inventionis shown in FIG. 7. First, at step S710, depending upon the drivingstate of a vehicle, an operation mode (either EV mode, HEV mode, or RBmode) is determined for the DCT. Then at steps S720, S730, and S740, theDCT is operated according to the determined operation mode.

A standard for determining the operation mode at step S710 may be set bya person of ordinary skill in the art. For example, the operation modemay be determined to be the RB mode when the HEV is braking orinertially running, to be the EV mode when a current SOC of the batteryis above a predetermined SOC when an accelerator pedal is operated, orto be the HEV mode when the current SOC of the battery is not above thepredetermined SOC when the accelerator pedal is operated.

At step S720 of operating the DCT in EV mode, the engine 102 is stoppedand the first and second clutches C1 and C2 are released while thetorque of the motor 103 is transmitted to a driven gear for apredetermined shift-speed (e.g., the second driven gear D2). In thiscase, only the torque of the motor from battery power is changed andoutput.

At step S730 of operating the DCT in HEV mode, the engine 102 is startedby the motor 103, either the first or second clutch (C1 or C2) isengaged such that the torque of the engine 102 is transmitted to adriven gear for a target shift-speed, and the torque of the motor 103 istransmitted to a second driven gear. In this case, the torque of themotor 103 is controlled based on current vehicle speed and depressionamount of an accelerator pedal.

At step S730 of operating the DCT in RB mode, the first and secondclutches C1 and C2 are released while a torque is transmitted from adriven gear for a predetermined shift-speed (e.g., the second drivengear D2) to the motor 103, such that the motor 103 is driven as anelectric generator by the braking and inertial energy of the HEV.

While the DCT operates in the various modes, the operation processrepeatedly returns to step S710 of determining the operation mode, andaccordingly the DCT may always be operated by an operation modeappropriate to a running state of the HEV.

According to an embodiment of the present invention, a DCT for an HEV isprovided with, in addition to an operation method thereof, a layout inpower transmission scheme and a motor location that are optimal forchanging and outputting torque of an engine and a motor. Therefore, sucha DCT may be used for an HEV, overcoming the inefficiency of aconventional CVT with a metal belt. By an application of such a DCT toan HEV, torque of an engine and a motor may be manipulated according tovarious operation modes. In addition, convenience of an automatictransmission may also be achieved by adapting two clutches and anautomatically shifting device to a scheme similar to a conventionalmanual transmission.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiments, but, on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims.

1. A method for controlling a double clutch transmission for a hybridelectric vehicle having an engine and a motor, the double clutchtransmission including: a main input shaft receiving torque from theengine; a first input shaft rotating coaxially with the main inputshaft; a second input shaft rotating coaxially with the main input shaftand along an exterior circumference of the first input shaft; first andsecond clutches for selectively transmitting torque of the main inputshaft to either the first or the second input shaft; a drive gear unitincluding a plurality of drive gears disposed on the first and secondinput shafts; a first output device including a first output shaftdisposed parallel to and apart from the first and second input shafts bya predetermined distance and also including a plurality of driven gearsand a first output gear thereon, such that torque of drive gears on thefirst and second input shafts are selectively changed and output; asecond output device including a second output shaft and a reverse idleshaft disposed parallel to and apart from the first and second inputshafts by predetermined distances, a plurality of driven gears, a secondoutput gear, a reverse driven gear disposed on the second output shaft,and a plurality of reverse mediating gears disposed on the reverse idleshaft, such that torque of drive gears on the first and second inputshafts are selectively changed and output; a differential gear connectedto both the first output gear and the second output gear; and a motorinput/output unit for selectively transmitting torque of the motor tothe second input shaft through a plurality of gears and a drive gear onthe second input shaft; the method comprising: determining an operationmode among an electric vehicle (EV) mode wherein only a torque of amotor is changed and output, a hybrid electric vehicle (HEV) modewherein both torque of the motor and an engine are changed and output,and a regenerative braking (RB) mode wherein braking and inertial energyof the hybrid electric vehicle is reclaimed by electric generation ofthe motor; and operating the double clutch transmission in accordancewith the determined operation mode.
 2. The method of claim 1, whereinthe determined mode is the EV mode when a current state of charge (SOC)is above a predetermined SOC, and wherein, in the EV mode, the engine isstopped and first and second clutches are released while torque from themotor is transmitted to a driven gear for a predetermined shift-speed,such that only torque from the motor from battery power is changed andoutput.
 3. The method of claim 2, wherein the predetermined shift-speedis a forward second speed.
 4. The method of claim 1, wherein, in the EVmode, power of the motor is output to a differential gear sequentiallythrough a motor gear, a motor idle gear, a second speed drive gear on asecond input shaft, a second speed driven gear on a first output shaft,and the first output shaft.
 5. The method of claim 1, wherein, in theHEV mode: the engine is started by the motor; one clutch of first andsecond clutches is engaged such that a torque of the engine istransmitted to a driven gear for a target shift-speed; a torque of themotor is transmitted to a second speed driven gear; and the torque ofthe motor is controlled on the basis of a current vehicle speed and adepression amount of an accelerator pedal.
 6. The method of claim 1,wherein the determined mode is the RB mode when the hybrid electricvehicle is decelerating by braking or is inertially running, andwherein, in the RB mode, first and second clutches are released whiletorque is transmitted from a driven gear for a predetermined shift-speedto the motor, such that the motor is driven as an electric generator bythe braking and inertial energy of the hybrid electric vehicle.
 7. Themethod of claim 1, wherein, in the RB mode, the braking and inertialenergy is input to the motor sequentially through a differential gear, afirst output gear, a first output shaft, a second speed driven gear onthe first output shaft, a second speed drive gear on a second inputshaft, a motor idle gear, and a motor gear.